/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlineGroupsInterface.h
Submit a request to join the specified group. This is done on behalf of the context user (provided when you request thisIOnlineGroupsinterface) and also updates the their cached membership info. You can call GetCachedUserMembership afterwards to see if they are a full member or petitioner.
void JoinGroup ( const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & ContextUserId, const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & GroupId, const [FOnGroupsRequestCompleted](API\Plugins\OnlineSubsystem\Interfaces\FOnGroupsRequestCompleted) & OnCompleted )